1. Field of Invention
The present invention relates to the manufacture of surface relief birefringent liquid crystal components. Such components are suitable for use in a wide range of applications, including without limitation autostereoscopic 3D displays, brightness enhanced displays, for use in illuminating display devices, particularly transmissive and transflective display systems and as a switching component for use in optical networking.
2. Description of Related Art
Surface relief birefringent optical components are described for example in WO-03/015424 and WO-2005/006056. A birefringent microlens array is formed from a surface relief interface between an isotropic material and an aligned birefringent liquid crystal material. Light of a first linear polarisation state passing through the device sees a first refractive index step at the surface relief interface between the isotropic material and the birefringent liquid crystal material, whereas light of a second orthogonal linear polarisation state sees a second, different refractive index step at the interface. A birefringent component of this type for a backlight is described for example in U.S. Pat. No. 5,751,388. A backlight system of this type for providing emission of polarised light is described in US-2003/0,058,383, in which disclosure a structured birefringent optical component is arranged to deflect light of a first polarisation and not deflect light of a second polarisation.
Birefringent liquid crystal components can be formed by means of a liquid crystal cell filling method as shown in FIG. 1. A rigid substrate 2 such as a glass or polymer substrate, has an isotropic polymer layer 4 formed on its surface by means of UV casting, embossing, thermal forming or other well known methods. The outer surface of the layer 4 is shaped with a surface relief structure and has provided thereon an alignment layer 6, for example polyimide. The alignment layer 6 may be formed for example by means of spin coating, printing or other known methods. The alignment layer 6 is cured, and rubbed to produce a directional liquid crystal alignment property. A second rigid substrate 8 with a second alignment layer 10 forms a cell gap between the alignment layers 6,10 which is capillary filled by birefringent liquid crystal material 12 as shown by arrow 14 typically at elevated temperature. The liquid crystal material 12 may be a curable liquid crystal material. In this case, following filling, the material is cured, for example thermally, by light or by electron beam radiation. Such materials allow high ruggedness, and can enable a reduction in the thickness of devices.
Such a capillary filling process has a number of difficulties. A lenticular surface with an array of elongate cylindrical lenses is a common surface relief structure. In this case, the capillary fill will often take place along the length of the lenses. However, these lenses may be susceptible to blockage, so that they do not fill uniformly, creating bubbles which degrade optical performance. In curable liquid crystal materials, bubbles may contain oxygen which inhibits cure of some types of polymerisable liquid crystal material. This can cause regions of strain in the cured material, degrading alignment properties of the liquid crystal material near the bubble.
More even filling can be achieved by incorporating a larger spacer gap between the alignment layers 6 and 10. However, such an approach disadvantageously uses more material, and so increases cost. Further, the uniformity of the thickness of additional material can be difficult to maintain, so the final device may not be flat, which may cause non-uniform optical output for example in an autostereoscopic display system.
During filling, a vacuum can be used to avoid the formation of air bubbles. Vacuum equipment is disadvantageously expensive, and the high levels of vacuum required for vacuum filling may not be compatible with the lens polymer materials.
The device further requires two substrates 2 and 8. Such substrates typically have a thickness of 0.4 mm or greater. The overall thickness of the display is thus large. To reduce thickness after fabrication, the present inventors have considered notionally removing the substrate 8 from the cured liquid crystal material 12. However, this is problematic. Removal of the rigid substrate 8 is difficult. If the substrate 8 is formed from glass, it may be prone to cracking. The surface energies of the interface between the liquid crystal material 12 and the alignment layers 6, 10 may be similar, so that delamination may take place unpredictably off either surface, therefore resulting in unreliability of delamination release. Further, the adhesion of the liquid crystal material 12 to the alignment layer 6 is required to be as high as possible, to maximise the endurance properties of the device. Higher surface energy may be achieved by addition of a wetting agent to the liquid crystal material 12. However, this may also increase the adhesion to the alignment layer 10, and thus reduce the reliability of delamination at the planar interface.
Further, the addition of alignment layer 10 adds cost to the processing method.
Further, the filling process can take some hours, particularly for a large cell required for large displays or for motherglass processing methods. Where the liquid crystal material 12 is a polymer liquid crystal it may be liable to thermal cure prior to cure by for example ultraviolet radiation. This means that such materials are difficult to use reliably in processes with prolonged process time. Premature cure may result in regions of non-uniform liquid crystal alignment and filling errors.
Another difficulty is that when birefringent components such as shown in FIG. 1 are manufactured in motherglass form, it is difficult to cut the motherglass and separate the individual components after processing in motherglass form. It is required to cut through the two different substrates 8, 2 as well as through the cured polymer layer 4 and the liquid crystal material 12 without causing delamination of the polymer material 4 or cured liquid crystal material 12.
It would be desirable to provide a method of manufacture of a surface relief birefringent liquid crystal component in which at least some of these difficulties are alleviated.